Preserving glyphosate in a Roundup Ready world

As new tillage programs and varieties have moved into Arkansas, corn production has become more complex. To describe Arkansas’ traditional weed control in corn, Ken Smith flashed a slide containing four words onto the screen: “Got atrazine? The End.”

“Arkansas has shifted to more conservation tillage,” said Smith, state Extension weed scientist at the recent Yell County corn production meeting in Dardanelle, Ark. “We still aren’t true no-till, for the most part. But reduced tillage has really increased. That means it’s even more obvious we can’t grow corn without herbicides and weed control.”

But one herbicide in particular — glyphosate — has Smith worried. “Glyphosate is extremely good on pigweeds. In cotton, we managed pigweed until we went to Roundup Ready cotton. Now, pigweed is just about to run us out of the cotton patch.

“That isn’t because we can’t kill it. Pigweed is still extremely susceptible, right now. But it just keeps on coming in waves. And pigweed has a tradition of becoming resistant to any herbicide we’ve treated it with for any long period.”

Spectrum shifted

The state’s weed spectrum has shifted, said Smith.

Asking the crowd if anyone has seen a cocklebur in the last five years, Smith saw some hands raised. “Not many. Everyone remembers how bad they used to be, though. Cocklebur used to be a serious problem — not so much anymore.

“I’m guessing around 60 percent of our corn will be Roundup Ready, next year. In the past, I’ve not been a huge proponent of Roundup Ready corn for a couple of reasons.

“First, I didn’t think the varieties were quite good enough. Now, though, the varieties available are looking much better. We have more varieties adapted to our part of the world.

The second reason: the threat of a glyphosate-resistant weed popping up. “If you’re planting Roundup Ready corn, I encourage you not to spray Roundup, Roundup, Roundup and walk away with a smile. First, you aren’t going to like that late-season weed control. We must tackle resistance management.”

Soapbox

Admitting this is his “soapbox,” Smith said, “glyphosate-resistant pigweed is a serious issue for us. We have built our agriculture around the glyphosate molecule — soybeans, cotton, and corn. Rice is coming. If we lose the glyphosate molecule, we’ll be in a world of hurt. Please keep that in mind when making spraying decisions in Roundup Ready corn.”

If a producer has horseweed in his field, Smith encourages spraying it with 8 ounces of Clarity in the spring.

“This isn’t because your horseweed is necessarily Roundup-resistant. (In west-central Arkansas) it may not be. But if it is, it’s going to cost a whole lot more to come back with a second application. Make sure and get the weed the first time.”

Horseweed isn’t terribly susceptible to Roundup to start with. “It’s a hard-to-kill weed. Regardless, we’ve always been able to knock it out with Roundup and a pint of 2,4-D. But when facing Roundup-resistant horseweed, the 2,4-D won’t kill it. When that happens, you’re in a worse situation than when you made the first spray.

“And let me be clear. I’m not talking about horseweed being a little bit tolerant or a little bit resistant. When this weed becomes resistant, it’s really resistant. When you spray resistant horseweed with 10 quarts of Roundup WeatherMax in the greenhouse and they don’t die, that’s pretty resistant.”

A lot of times when spraying resistant horseweed with Roundup — particularly at higher rates — the terminal will be taken out. That allows the plant to branch and allows an even larger seed-producer to emerge. Horseweed produces about 200,000 seeds per plant “and they’ll blow everywhere.”

Atrazine

On Roundup Ready corn, “remember the 12-inch cut-off on atrazine. Lexar and Steadfast ATZ both contain atrazine and also must adhere to that 12-inch cut-off. I certainly encourage you to put atrazine in your Roundup Ready program. Another cut-off is for Callisto and Steadfast at 24 inches.”

Callisto does “almost as good” a job as atrazine, said Smith. The reason Callisto, and other products, are on the market is because “EPA absolutely doesn’t like atrazine. These products are being developed and pushed as a replacement for atrazine, although many aren’t as cheap. The EPA would love to ban atrazine. But there’s so much of it used, the political side of things have come into play and they haven’t been able to.”